Add python api for argsort_op

Author: Yibing Liu

doc/fluid/api/layers.rst

@@ -1105,6 +1105,12 @@ argmax
 ..  autofunction:: paddle.fluid.layers.argmax
     :noindex:
 
+argsort
+------
+
+..  autofunction:: paddle.fluid.layers.argsort
+    :noindex:
+
 ones
 ----
 

paddle/fluid/operators/argsort_op.cc

@@ -34,13 +34,13 @@ class ArgsortOp : public framework::OperatorWithKernel {
 
     auto num_dims = in_dims.size();
     PADDLE_ENFORCE(axis < num_dims,
-                   "Attr(axis) %d of ArgsortOp is out of bounds for Input(X) "
-                   "dimension %d.",
+                   "Attr(axis) %d of ArgsortOp is out of bounds for Input(X)'s "
+                   "rank %d.",
+                   axis, num_dims);
+    PADDLE_ENFORCE(axis >= -num_dims,
+                   "Attr(axis) %d of ArgsortOp must be not less than "
+                   "-rank(Input(X)) (%d).",
                    axis, num_dims);
-    PADDLE_ENFORCE(in_dims.size() + axis >= 0,
-                   "Attr(axis) %d of ArgsortOp plus the rank %d of Input(X) "
-                   "must be nonnegative.",
-                   axis, in_dims.size());
 
     ctx->SetOutputDim("Out", in_dims);
     ctx->SetOutputDim("Indices", in_dims);

python/paddle/fluid/layers/tensor.py

@@ -33,6 +33,7 @@
     'fill_constant',
     'argmin',
     'argmax',
+    'argsort',
     'ones',
     'zeros',
     'reverse',
@@ -438,6 +439,56 @@ def argmax(x, axis=0):
     return out
 
 
+def argsort(input, axis=-1):
+    """
+    Performs sorting on the input Variable along the given axis, and outputs 
+    sorted data Varibale and its corresponding index Variable with the same 
+    shape as :attr:`input`.
+
+    .. code-block:: text
+    
+        For example, the given axis is -1 and the input Variable
+
+            input = [[0.15849551, 0.45865775, 0.8563702 ],
+                     [0.12070083, 0.28766365, 0.18776911]],
+
+        after argsort, the sorted Vairable becomes
+
+            out = [[0.15849551, 0.45865775, 0.8563702 ],
+                   [0.12070083, 0.18776911, 0.28766365]],
+
+        and the sorted indices along the given axis turn outs to be
+
+            indices = [[0, 1, 2], 
+                       [0, 2, 1]]
+
+    Args:
+        input(Variable): The input Variable for sorting.
+        axis(int): The axis along which to sort the input Variable. When 
+                   :attr:`axis` < 0, the actual axis will be :attr:`axis` + 
+                   rank(:attr:`input`). Default -1, the last dimension.
+
+    Returns:
+        tuple: A tuple of sorted data Variable and the sorted indices.
+
+    Examples:
+        .. code-block:: python
+
+            input = fluid.layers.data(data=[2, 3])
+            out, indices = fluid.layers.argsort(input, axis=0)
+    """
+    helper = LayerHelper("argsort", **locals())
+    out = helper.create_tmp_variable(dtype=input.dtype, stop_gradient=True)
+    ids = helper.create_tmp_variable(VarDesc.VarType.INT64, stop_gradient=True)
+    helper.append_op(
+        type='argsort',
+        inputs={'X': input},
+        outputs={'Out': out,
+                 'Indics': ids},
+        attts={'axis': axis})
+    return out, ids
+
+
 def ones(shape, dtype, force_cpu=False):
     """
     **ones**